1. Field of the Invention
The present invention relates generally to improvements in a lens antenna which comprises a dielectric lens attached to an aperture of a horn, and more specifically to a lens antenna which includes an improved dielectric lens for effectively lowering disturbances caused by electromagnetic waves internally reflected in the lens.
2. Description of the Related Art
As is known in the art, a lens antenna is comprised of a dielectric lens secured at an aperture (mouth) of a horn. The dielectric lens functions as a wave collimating element. A lens antenna is typically used in line-of-sight terrestrial microwave communications systems.
Before turning to the present invention it is deemed preferable to describe a known lens antenna with reference to FIG. 1.
FIG. 1 is a side view, partly sectional, of a known lens antenna, generally denoted by numeral 10, which comprises a plano-convex dielectric lens 12 and a conical horn 14 serving as a flared-out waveguide. The plano-convex lens 12 is made of a dielectric material such as polyethylene, polystyrene, etc. with a relative permittivity ranging about from 2 to 4. The lens 12 has plane surface 16 facing a free space and a hyperboloid of revolution (denoted by numeral 18) at the inner side. The horn 14 has a circular aperture to which the lens 12 is secured at its periphery. The horn 14 has an inner well covered with an electrically conductive layer, and has a flange 20 to which a corresponding flange 22 of a waveguide member 24 is attached. Reference numeral 26 denotes a wave guide.
As is well known in the art, the lens 14 transforms the spherical wave front of the wave radiated from a source 28 (i.e., primary antenna) into a plane wave front. To be more explicit, the field (viz., electromagnetic field) over the plans surface (viz., plans wave front) can be made everywhere in phase by shaping the lens so that all paths from the wave source 28 to the lens plane are of equal electrical length (Fermat's principle).
As shown in FIG. 1, part of a given incident wave 28 is reflected at two points of the lens 12: at the convex surface 18 (the reflected component is indicated by a broken line arrow 29) and at the plane surface 18. The reflection from the convex surface 18 does not return to the source 28 except from points at or near an axis 32 and thus are of no consequence. However, the energy reflected from the lens plans 16 returns back exactly along the radiation line 30 and may adversely affect the energy to be radiated from the wave source 26.
It is therefore highly desirable to reduce the above mentioned undesirable influence caused by the reflections from the plane lens surface.